Hydrocracking of hydrocarbon feedstocks is often used to convert lower value hydrocarbon fractions into higher value products, such as conversion of vacuum gas oil (VGO) feedstocks to various fuels and lubricants. Typical hydrocracking reaction schemes can include an initial hydrotreatment step, a hydrocracking step, and a post hydrotreatment step, such as dewaxing or hydrofinishing. After these steps, the effluent can be fractionated to separate out a desired lubricant oil basestock.
One of the difficulties in lubricant base oil production is that hydroprocessing a lubricant feed to achieve desired lubricant base oil properties also results in conversion of a feed. The majority of molecules in a typical lubricant base oil have a boiling point greater than 370° C. In order to achieve desired properties, such as a reduced sulfur content or a higher viscosity index (VI), a feedstock is hydrotreated and/or hydrocracked to improve the feed properties. However, the improvement of feed properties is accompanied by conversion of a portion of the feed to molecules with a boiling point below 370° C. This results in a loss of yield for the lubricant base oil, as the converted molecules are more appropriate for use as a fuel.
European Patent EP 0471461 describes a method for producing low pour point and high viscosity index lubricant base oils by using solvent dewaxing. A low boiling waxy oil with a conventional viscosity index is blended with a high viscosity index, higher boiling oil. This blended oil is then dewaxed to a desired pour point. Because of the difference in boiling points, the low boiling waxy oil can then be separated out from higher boiling oil. The yield for the high viscosity index, higher boiling oil after solvent dewaxing of the blended oil to a desired pour point is increased relative to performing solvent dewaxing to the same pour point on only the higher boiling fraction.
U.S. Pat. No. 7,708,878 describes a system and method for generating lubricant base oils. After hydroprocessing of a feed, the hydroprocessed feed is subjected to two fractionations. A first portion of the hydroprocessed feed is fractionated in light block mode operation to produce a first set of base oil fractions. A second portion of the hydroprocessed feed is fractionated in medium block mode operation to produce a second set of base oil fractions. The first set and second set of base oil fractions can then be used to form various lubricant base oil products via blending.